The present invention relates to a pneumatic machine tool particularly suited to use in the aeronautical industry.
Pneumatic machines are widespread and often preferred over electrically driven machines because of their performance.
The prior art already teaches a pneumatic machine tool of the type comprising a telescopic tool-holder spindle comprising a drive shaft driven in rotation by a pneumatic motor and on which is slidingly mounted a hollow shaft with which a tool holder is associated and which is driven in translation by a control means comprising a carriage driven in translation by an endless screw via a nut carried by the carriage, the said endless screw being driven in rotation by an electric motor.
This type of machine tool is often fitted with a ring allowing the machine tool to be locked to a machining rack.
Once the machine tool has been mounted on this rack, a central processing unit with which the machine tool is equipped actuates the electric motor so as to begin a phase of bringing the tool quickly up into contact with the workpiece that is to be machined.
This phase continues in the form of a machining phase proper. Finally, after machining, the cycle is finished.
To date, there are various techniques that enable the end of machining to be detected.
In particular, detection of drilling may be achieved by measuring the strength of the current drawn by the electric motor. This technique is ineffective in certain instances in so far as it cannot be used when the electric motor is a motor of the stepping type.
It is also possible to detect the end of drilling of the workpiece by detecting an increase in the rotational speed of the drive shaft of the machine tool. This technique has a certain number of drawbacks particularly as a result of the fact that it is unable to supply precise information in so far as the rotational speed of the shaft varies according to a great many parameters, for example according to the amount of lubricant on the workpiece.
The object of the invention is to alleviate the drawbacks of the state of the art.
The subject of the invention is therefore a pneumatic machine tool comprising a telescopic spindle having a drive shaft driven in rotation by a pneumatic motor and on which is slidingly mounted a hollow shaft with which a tool holder is associated and which is driven in translation by control means comprising a carriage driven in translation by an endless screw via a nut carried by the carriage, the said endless screw being driven in rotation by an electric motor, characterized in that the carriage is provided with a force sensor designed to measure the thrust exerted on the tool holder and connected to a central processing unit in which is stored an algorithm for processing the signal delivered by the sensor and for monitoring the advance of the tool carried by the spindle.
The pneumatic machine tool according to the invention may furthermore have one or more of the following features, taken in isolation or in any technically feasible combination:
the processing algorithm comprises software means for calculating the difference between the signal delivered by the sensor and a thrust threshold value corresponding to a minimum value for the thrust exerted during the machining of a workpiece,
the calculation means constitute means of detecting the progress of the drilling of a workpiece in the course of machining,
the electric motor is an electric stepping motor, the machine tool further comprising a counter for measuring the axial displacement of the tool and the central processing unit controlling the stopping of a machining cycle in response to detection of the end of drilling as soon as, after drilling, the tool has advanced by a predetermined distance,
the sensor is mounted between a member for immobilizing the nut on the carriage and the nut,
the sensor is incorporated into a member for holding the nut on the carriage,
the sensor is mounted between a holding member and the carriage,
the sensor consists of a strain gauge,
the sensor consists of a piezoelectric sensor,
the sensor is mounted in a Wheatstone bridge.